Article 7
RT&S
Ask John
Where and what should I be looking for when inspecting a timber bridge?
This response deserves an entire chapter, but here is a “Readers Digest” version. Consider those areas of
the bridge where holes have been drilled for drift pins or bolts, bearing areas, piling around the ground line
and at the water elevations, and top surfaces exposed to wet/dry cycles.
Stringers – The stringers have two bearing areas; under the ties and over the caps. Their top surfaces
between the ties (open deck structures) are subject to weather deterioration and they have vertical holes
over the caps for drift pins, vertical holes for line bolts (securing the ties), and usually have horizontal holes
for the chord bolts.
Caps – The caps also have two bearing areas; under the stringers and over the piles. The caps have
vertical holes for the drift pins and possibly line bolts for the ties. If the bents have cross bracing, the caps
will have horizontal holes secure the bracing
Piles – The piles will have vertical holes under the cap for the drift pin and horizontal holes for any cross
bracing. Each pile is especially subject to decay at the ground line or at the waterline, especially if the
waterline fluctuates. In addition to the drift pins at the top of the piles, this is also a bearing area under the
cap.
Deterioration of timber components generally does not occur in solid sections. The main exception to this
comment is at the ground line or water line of piles. Each of the above conditions describes locations where
timber decay normally begins, and the decay will begin inside of the section as a void and will grow over the
years. As these voids grow over the years, the thickness of the remaining timber between the void and
surface becomes smaller and is called the “shell thickness.”
When inspecting a timber bridge, it is beneficial to use a hammer, or other sounding tool, to “sound” the
timber components at each of the above noted locations. If you hear a sound like a hollow pumpkin when
you hit the timber components with your sounding tool, it is a good indication that the void area is getting
large and the shell thickness is small. Detailed inspections will incorporate a drilling procedure to physically
measure the shell and void sizes. If the void becomes too big and the shell thickness too small, the timber
component will eventually fail. These failures can be gradual and become obvious with a visual inspection,
or they can be a sudden failure under a live load.
Using these inspection procedures with a focus on the noted areas, you will have gone a long way in finding
any serious problems with your timber bridge.